Modern telecommunications systems transmit, receive, store and retrieve ever increasing amounts of data. The transmission of information between ICs in complex systems requires that the signaling scheme and input-output (I/O) circuitry be capable of high speed operation, generate minimal disturbances (noise), be tolerant to interference, consume little power and occupy a minimum area on the IC. Furthermore, it would be highly desirable from a usability point of view that the I/O circuitry support different supply voltages in the transmitter and receiver and be capable of multi-mode operation so as to enhance both backward and forward compatibility with ICs of different generations that may use different signaling schemes.
Conventional I/O circuit cells typically support only one type of signal, for example single-ended CMOS signals, and the supply voltage of the transmitting cell must be the same as the supply voltage of the receiving cell, and vice versa. Some conventional I/O cells can be used as transmitters or receivers to support bi-directional signaling.
The most commonly used CMOS digital signaling techniques use single-ended voltage mode signals with rail-to-rail levels and fast edges. However, this approach is known to generate a significant amount of signal disturbance and interference with other circuitry, and tends to limit the maximum usable data rates and/or seriously affects the performance of the system. The generated disturbances are especially detrimental in radio-based communications systems, where received analog signals can be extremely weak.
One technique to reduce the generation of disturbance signals is to use analog signaling between ICs. This approach implies that both the transmitter and receiver ICs must contain analog circuitry, e.g., analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). However, the incorporation of any analog circuitry into an otherwise digital IC is problematic as digital ICs are typically implemented using highly optimized digital processes. In these processes the analog properties of devices are often compromised. In addition, the maximum tolerable supply voltage of these deep sub-micron processes is decreasing, which makes the implementation of analog functions increasingly difficult. In addition, the variety of available analog devices is limited. For example, passive devices, such as resistors, are made available only through the use of expensive additional process steps. Furthermore, analog signaling and the associated analog circuitry (e.g., ADCs and DACs) in digital ICs may lead to a prohibitively large circuit area requirement, as well as to an unacceptable power consumption. Also, implementing analog or mixed signal circuitry on large digital ICs makes design and testing more difficult, time consuming and expensive, and therefore increases both risk and delay.
Moreover, the price per silicon area in deep sub-micron CMOS processes is increasing. As the area of analog circuitry does not scale down at the same rate as digital circuitry, the placement of analog circuitry on digital ICs becomes increasingly expensive.
From the above it can be appreciated that it is advantageous that large digital ICs contain only digital circuitry, and that RF, analog and mixed signal circuitry is preferably placed in a separate chip that is implemented using a more appropriate process technology. This being the case, it can further be appreciated that the signaling between ICs should be optimized instead, without relying on the use of analog-based I/O circuitry.
It is thus important to develop efficient inter-IC signaling circuitry that enables an optimum system partitioning to be achieved. Prior to this invention, this need has not been adequately addressed.